if i might suggest one way to build up to a conference is to run meetups beforehand. not many are needed, say 3-6 of them before you probably have the community trust and events experience to go big.
I think it’s still running even after I moved out of the country 8 years ago.
edit: Our venue trick was “Hey, if we pack your bar/restaurant on a random weekday night so you make a killing in drinks, can we have the place?”. Venues were happy to say yes.
They all need bars to dance. They all use the same argument. And quite often they are politely asked to find somewhere else after a few months.
PS - the issue is that many social dancers want to dance over being social, and alcohol detracts from it. People have jobs and want to pay to contribute. We ended up settling on a cover charge that comes with food credit so people can booze up if desired, and buy water/snacks if not. Win-win.
And if you packed their busiest night, they might make some more money but it might cause them problems and stress them out. Offer to boost their deadest night.
Like you said, just talk to the manager and say "hey I've got 50 people that want a place to hang out and order beer. Can we do that at your place or should we go somewhere else?". Easy win for everyone.
Running a conference is harder. Hotel conference rooms are way over priced. The AV prices are morally offensive.
If your attendance is under 100 you can probably do a restaurant. It will cost food and beverage but you likely just need to hit a $ minimum. Fees should be minimal. If you need conference space for over 100 I don't think there are any "good" options. And definitely no good options if you get into several hundred.